1. Download the .zip-file in the first post.2. Place the file in the SuperTux-Addon directory (in Linux it is YourDirectory/.local/share/supertux2/addons/, in Windows search for %appdata% and then for supertux2 or something). DONT unzip it.3. Then start SuperTux and go to the Add-ons. Check the box for "World 'Desert World' by 'manuel' "4. Now you should find it in the Contrib Level section